High density electrical contacts



nited States Patent Oil 4 Claims ABSTRACT OF THE DISCLOSURE The invention provides electrical contact members especially adapted for heavy duty service consisting essentially of nickel and titanium carbide together with (1) a metal of the group silver and copper, and (2) a metal of the group molybdenum and tungsten. In the preferred embodiment of the invention the contacts consist essentially of, by volume, 15 to 60 percent of silver, 30 to 55 percent of titanium carbide, 8 to 40 percent of nickel, and 0.1 to 10 percent of molybdenum. The cont-acts are made by pilling those constituents and then sintering the compacts, and they are characterized by the fact that their density is at least 95 percent of the theoretical density.

This application is related to my copending application Ser. No. 518,557, filed Jan. 4, 1966 now abandoned.

This invention relates particularly to electrical contacts especially suitable for heavy duty use in, for example, circuit breakers. Such devices are frequently used as switches, for instance in to ampere service. For this use the contacts are commonly of silver-tungsten, silver-molybdenum or copper-tungsten. When circuit ice tions even though they might be supposed to behave rather similarly to the Cu-Ni-W compositions just mentioned. For example, the literature shows that such compositions as silver-tungsten and copper-tungsten cannot be pressed and sintered to density even approaching 90 percent of theoretical; the same thing is true for molybdenum based contacts, as well as for contacts in which the copper in the Cu-Ni-W would be replaced by silver.

In general, then, it is true that in the known contact systems for heavy duty use density close to theoretical is not ordinarily attainable directly by sintering such pilled compositions. It might be thought that the goal of maximum density could be had by the application of sufficiently high pilling pressure, the idea being that thereby porosity could be minimized with concomitant increase of density. Experience has shown, however, that although the density of compacts may increase with pilling pressure increase, a point is reached beyond which the density tends to decrease. Increased pressure is furthermore unjustified by the cost of the sufliciently strong dies that would be needed and the power required. Furthermore, even if one were to pill at unusually high pressures, there would then be the very strong liklihood that the compact would disintegrate during sintering due to the evolution of gases which had been trapped on the outer surfaces of the powder particles.

As exemplifying this situation, the following table represents the results of tests in which the compositions listed were pilled under the conditions stated and then sintered at 1600 C. The table lists also the extent of shrinkage during the sintering step and the density of the sintered compacts in terms of the percent of theoretical density.

Molded Sintered Percent Ag Th. Percent Percent n den., Den., Th. Percent Th. MlX g./ce. T.s.1 g./cc. den. Cycle Shrink. Den.

Type Mix:

Ag-Mo 44 10.3 7. 7 75 15" at 1,600 O 1. 5 78 Ag- 48 13.8 40 11.4 82. 5 do 1.3 85 Ag-W 40 14. 5 4O 11. 2 1. 0 79 Ag-W 15 17 40 12. 5 0.3 73

breakers provided with such contacts are used as switches with frequently repeated make-and-break operation there is a tendency for the contacts to become heated with consequent damage to the contact surfaces, and the temperature rise may even be great enough to result in catastrophic failure.

It is recognized that the best performance of electrical contacts is to be had with those the density of which approaches theoretical, i.e., at least 95 percent of theoretical, and more suitably higher.

Acceptable density can be obtained by infiltrating the compacts, as with silver, or by hot pressing or hot coining. Hot pressing and hot coining are not looked upon with favor by the manufacturers of these articles because of the unfavorable economics of those procedures. For the same reason the trade would like to eliminate the infiltration step. However, those practices have been followed down to the present time because it has not been possible to achieve the desired density by simple pilling and sintering of the most desirable compositions.

Cu-Ni-W contact compositions can be made at close to theoretical density by pilling and sintering for prolonged lengths of time, of the order of hours, and the same thing is true of a few other compositions, but the lengthy sintering needed is an economic burden. Oddly enough that practice does not suffice for most other contact composi- The data of the table show that the densities of pilled and sintered contacts of the compositions commonly used as heavy duty contact materials are substantially below the minimum of theoretical) believed to be essential for acceptable performance.

There is no accepted explanation for the anomalogous behaviors of contact materials as just described, and the field of contact manufacture remains an art in the absense of a sound basis of understanding the differences in behavior of the various compositions and the general inability to achieve the desired degree of density by simple pilling and sintering.

A major object of this invention is to provide electrical contacts particularly adapted for the type of service alluded to above, which are characterized by having density at least and preferably 98 percent of theoretical, and which can be made without the usual step of infiltrating sintered compacts.

Another object is to provide such contacts characterized by especially low temperature rise in use, especially in frequently repeated make-and-break operation, which are of less expensive composition than the contacts conventionally used for such service, and which may be made easily by powder metallurgy methods without using the infiltration step, hot pressing or hot coining such as have been necessary up to the present time in order to produce acceptable maximum density with the most favored compositions.

In the preferred embodiment of the invention the contacts consist essentially of, by volume:

Percent Silver 15-60 Titanium carbide 30-55 Nickel 8-40 Molybdenum 0.1-10

Within that range a preferred composition consists of, by volume, 30 percent of silver, 46 percent of titanium carbide, 19 percent of nickel, and percent of molybdenum. Furthermore, optimum results are achieved when the volume percentage of silver plus nickel is in the range 40 to 70 percent.

In the practice of this invention the preferred embodiment just described may be modified by substituting copper for the silver, or tungsten for the molybdenum, or both, in the ranges for silver and molybdenum just stated, so that the invention includes the systems AgNiMoTiC, CuNiMoTiC, AgNiWTiC and CuNiWTiC.

Compositions within the foregoing ranges give good performance in the type of service contemplated, and perform satisfactorily in all phases of the Underwriters Laboratories tests. High contents of nickel are unusual in contacts because they do not contribute to electrical conductivity or to suppress welding, but these contacts perform well even so. But compositions outside of these ranges have been found to be unsatisfactory. From an economic standpoint the compositions of this invention are important also because the amounts of silver and molybdenum used, which are expensive materials, are significantly reduced beneath the levels commonly required in silver-tungsten and silver-molybdenum contact grades. As far as I am aware, there are no commercially useful contact materials for circuit breaker applications in the 20-ampere and above range which contain nickel to the extent used in the present invention; it would normally to expected that such high nickel contents would cause short circuit failure but experience has shown that the contacts of this invention give excellent short circuit performance.

A unique property of these new contacts is that the sintered porosity is independent of molding pressure, i.e. the compacts shrink to approximately the theoretical density regardless of the green density.

These contacts are made by powder metallurgy procedures that are so Well known and established that those familiar with that art will know how to practice this invention from the following illustrative procedure.

A blend of, by weight, 40 percent of silver, 30 percent of titanium carbide, 22.5 percent of nickel, and 7.5 percent of molybdenum, all in the form of minus 325 mesh powders is lubricated with a suitable binder to facilitate pilling, as by adding a dry wax to the dry powders. The wax and the powder blend are thoroughly mixed and the mixture is pilled at tons per square inch. The contact is sintered 5 minutes in a non-oxidizing atmosphere at 1375 C. The density of the product is in excess of 95 percent of theoretical, i.e. 7.4 to 7.5 g./cc. In this case the theoretical density was 7.6 g./cc., the molded density was 5.5 g./cc. (73% of theoretical), and the shrinkage due to sintering was 17%. These results are to be compared with those of the foregoing tabulation.

The superiority of the contacts of this invention in comparison with the presently available commercial contacts for service of the type contemplated is demonstrable by a test used by some circuit breaker manufacturers for evaluating contacts for such service. In this test the breakers are subjected to continuous make-and-break action for 1,000 cycles after which they are left in the make condition until the temperature has equilibrated, normally about two to three hours, and the temperature rise above ambient is determined. This operation is repeated for six cycles of 1,000 operations each. It is desired that the maximum temperature rise in these tests be 50 C.

As showing the superior character of contacts made in accordance with the present invention in comparison with contacts presently used commercially, reference may be made to actual comparative results of applying the test procedure just described using circuit breakers supplied by two manufacturers with determination of the resulting temperature rises, after which the contacts supplied by the manufacturers were replaced with contacts in accordance with the preferred embodiment of this invention, again with determination of the temperature rises. The results follow:

MANUFACTURER NO. 1

Standard equipment contacts C. Cycle 1, 4 tests 63-97 Cycle 2, 4 tests 90-115 Cycle 3, 4 tests -120 Cycle 4, 4 tests 67-90 Cycle 5, 4 tests 85-120 Cycle 6, 4 tests "a 75-115 Contacts this invention Cycle 1, 2 tests 15-18 Cycle 2, 2 tests 18-25 Cycle 3, 2 tests 20-25 Cycle 4, 2 tests 70 Cycle 5, 2 tests 55-62 Cycle 6, 2 tests 55-60 MANUFACTURER NO. 2 Standard equipment contacts C. Cycle 1, 4 tests 8-14 Cycle 2, 4 tests 20-50 Cycle 3, 1 test 28 Cycle 3, 3 tests 80-90 Cycle 4, 1 test 38 Cycle 4, 3 tests 80-115 Cycle 5, 4 tests 70-110 Cycle 6, 4 tests 73-96 Contacts this invention Cycle 1, 4 tests 15-37 Cycle 2, 2 tests 15-20 Cycle 2, 2 tests 63-70 Cycle 3, 4 tests 15-35 Cycle 4, 4 tests 16-46 Cycle 5, 3 tests 15-32 Cycle 5, 1 test 65 Cycle 6, 2 tests 15-25 Cycle 6, 2 tests 55-67 From the foregoing tests it will be noted that in the case of both circuit breakers the mean temperature rise was appreciably lower with the contacts of this invention. Also, in the case of the No. 1 standard breaker temperatures capable of causing catastrophic failure were reached in every case, while such temperatures were reached in four instances with the No. 2 standard breaker. In contrast with the contacts of this invention the maximum temperature was only 70 C. and in most instances it was below the desired maximum of 50 C. Furthermore, the contacts in accordance with the invention exhibited a stronger tendency to repair themselves in use and resist catastrophic failure, that is, temperatures in excess of 75 C. above ambient.

I claim:

1. An electric contact consisting essentially of, by volume, a sintered body of about '15 to 60 percent of a metal of the group copper and silver, about 30 to 55 percent of titanium carbide, about .8 .to 40 percent of nickel, and about 0.1 to 10 percent of a metal of the group tungsten and molybdenum, thedensity of the sintered contact being at least about percent of theoretical.

2. A contact according to claim 1, consisting essentially of, by volume, about percent of silver, 46 percent of titanium carbide, 19 percent of nickel, and 5 percent of molybdenum, the density of the sintered contact being at least 95 percent of theoretical.

3. A contact according to claim 1, in which the amounts of copper and tungsten, are as stated in claim 1.

4. A contact in accordance with claim 1, the volume percentage of silver plus nickel being in the range to percent.

References Cited UNITED STATES PATENTS 2/1939 Emmert 173 OTHER REFERENCES 5 Goetzel: Treatise on Powder Metallurgy, vol, II, 1950,

Interscience Publishers, New York, London, p. 183.

CARL D. QUARFORTH, Primary Examiner 10 ARTHUR J. STEINER, Assistant Examiner US. Cl. X.R. 

